A prior art search was conducted in the public records of the U.S. Patent and Trademark Office and disclosed the following references:
U.S. Pat. No. 3,866,025 is a digital three axis attitude controller, but is not a true PWPF (Pulse Width Pulse Frequency) system as in the present invention: each pulse train has a variable number of pulses each having a fixed pulse width, whereas the present invention varies the width as well as the frequency of pulses in response to spacecraft dynamics and to control same. The cited patent does not employ shift registers as in the present invention. It discloses a system mainly for precessing the angular momentum vector of the spacecraft, without the capability to perform station-keeping and control spacecraft attitude simultaneously as in the present invention. It requires expensive rate integrating gyros in addition to sun sensors and/or earth sensors, whereas the present invention does not require rate integrating gyros. The cited reference combines information from all axes at once, while the present invention operates upon the axes independently with simple uncoupled control laws.
U.S. Pat. No. 3,937,423 discloses a technique for nutation damping while maintaining attitude control with respect to just the roll axis; the present invention, on the other hand, controls the attitude along each of three orthogonal axes. The cited reference is an analog signal processing technique, whereas the present invention, in its preferred embodiment, is a wholly digital system, offering concomitant advantages. The cited patent issues thruster pulses timed according to the spacecraft nutation period, whereas the present invention does not phase the thruster firings with nutation periods, and has a quasi-real-time response to fast spacecraft dynamics that is much shorter than the nutation period. The cited patent pertains to on-orbit control whereas the present invention has a broader range of applicability encompassing acquisitions, station-keeping, or any other maneuver that requires thruster controls. The cited patent does not produce pulses of variable widths and is therefore not a PWPF system as in the present invention.
U.S. Pat. No. 3,944,172 is an attitude control system which could possibly be extended to three axis control, although just one axis is described. It is not a PWPF system because just the pulse width, and not pulse frequency is varied within a sampling period. The cited patent does not employ shift registers, which the present invention advantageously uses to mete out the thruster command signals while freeing up its processor (2) to perform other spacecraft functions.
U.S. Pat. Nos. 3,643,897 and 4,161,780 disclose techniques associated with spin stabilization, and not three axis spacecraft control.